The present invention relates to a receiver having a field effect transistor (FET) frequency mixer, and more particularly to a circuit for controlling the frequency conversion gain in an FET frequency mixer circuit.
To stabilize the level of a received signal wave, a high frequency amplifier with automatic gain control (AGC) has been used. If it is not possible to achieve a sufficiently wide AGC dynamic range by the use of only the AGC high frequency amplifier, the use of a high frequency mixer with AGC function is conceivable. The AGC function can be obtained by, for instance, controlling the gate voltage of a dual gate FET mixer. This gate voltage control, however, is usually operated at a local input level of around 1 V. In this case, even if the D.C. voltage of the gate is a pinch-off voltage, there will be biasing at the local input level, resulting in the failure of the FET to be pinched off. Therefore, the dynamic range of the conversion gain MG.sub.T of the mixer cannot be readily expanded merely by controlling the gate voltage of the FET and, moreover, this difficulty is accompanied by the instability of the mixer circuit owing to the variation of the D.C. biasing. For details of a mixer using an FET, reference is made to the U.S. Pat. No. 4,112,373 issued on Sept. 5, 1978.